A domestic yak infected with bluetongue virus. Tongue is swollen, cyanotic, and protruding from the mouth.

In sheep, BTV causes an acute disease with high morbidity and mortality. BTV also infects goats, cattle and other domestic animals as well as wild ruminants (for example, blesbuck, white-tailed deer, elk, and pronghorn antelope).[2]

Major signs are high fever, excessive salivation, swelling of the face and tongue and cyanosis of the tongue. Swelling of the lips and tongue gives the tongue its typical blue appearance, though this sign is confined to a minority of the animals. Nasal symptoms may be prominent, with nasal discharge and stertorous respiration.

Some animals also develop foot lesions, beginning with coronitis, with consequent lameness. In sheep, this can lead to knee-walking. In cattle, constant changing of position of the feet gives bluetongue the nickname The Dancing Disease.[3] Torsion of the neck (opisthotonos or torticollis) is observed in severely affected animals.

Not all animals develop symptoms, but all those that do lose condition rapidly, and the sickest die within a week. For affected animals which do not die, recovery is very slow, lasting several months.

The incubation period is 5–20 days, and all symptoms usually develop within a month. The mortality rate is normally low, but it is high in susceptible breeds of sheep. In Africa, local breeds of sheep may have no mortality, but in imported breeds it may be up to 90 percent.[4]

In cattle, goats and wild ruminants infection is usually asymptomatic despite high virus levels in blood. Red deer are an exception, and in them the disease may be as acute as in sheep.[5]

The virus particle consists of ten strands of double-stranded RNA surrounded by two protein shells. Unlike other arboviruses, BTV lacks a lipid envelope. The particle has a diameter of 86 nm.[7] The structure of the 70 nm core was determined in 1998 and was at the time the largest atomic structure to be solved.[8]

The two outer capsid proteins, VP2 and VP5, mediate attachment and penetration of BTV into the target cell. The virus makes initial contact with the cell with VP2, triggering receptor-mediated endocytosis of the virus. The low pH within the endosome then triggers BTV's membrane penetration protein VP5 to undergo a conformational change that disrupts the endosomal membrane.[7] Uncoating yields a transcriptionally active 470S core particle which is composed of two major proteins VP7 and VP3, and the three minor proteins VP1, VP4 and VP6 in addition to the dsRNA genome. There is no evidence that any trace of the outer capsid remains associated with these cores, as has been described for reovirus. The cores may be further uncoated to form 390S subcore particles that lack VP7, also in contrast to reovirus. Subviral particles are probably akin to cores derived in vitro from virions by physical or proteolytic treatments that remove the outer capsid and causes activation of the BTV transcriptase. In addition to the seven structural proteins, three non-structural (NS) proteins, NS1, NS2, NS3 (and a related NS3A) are synthesised in BTV-infected cells. Of these, NS3/NS3A is involved in the egress of the progeny virus. The two remaining non-structural proteins, NS1 and NS2, are produced at high levels in the cytoplasm and are believed to be involved in virus replication, assembly and morphogenesis.[2]

The molecular epidemiology of bluetongue virus (BTV) in Europe since 1998: routes of introduction of different serotypes and individual virus strains.

Bluetongue has been observed in Australia, the USA, Africa, the Middle East, Asia and Europe.

Its occurrence is seasonal in the affected Mediterranean countries, subsiding when temperatures drop and hard frosts kill the adult midge vectors.[9] Viral survival and vector longevity is seen during milder winters.[10] A significant contribution to the northward spread of Bluetongue disease has been the ability of Culicoides obsoletus and C.pulicaris to acquire and transmit the disease, both of which are spread widely throughout Europe. This is in contrast to the original C.imicola vector which is limited to North Africa and the Mediterranean. The relatively recent novel vector has facilitated a far more rapid spread than the simple expansion of habitats North through global warming.[citation needed]

In August 2006, cases of bluetongue were found in the Netherlands, then Belgium, Germany, and Luxembourg.[11][12] In 2007, the first case of bluetongue in the Czech Republic was detected in one bull near Cheb at the Czech-German border.[13] In September 2007, the UK reported its first ever suspected case of the disease, in a Highland cow on a rare breeds farm near Ipswich, Suffolk.[14] Since then the virus has spread from cattle to sheep in Britain.[15] By October 2007 bluetongue had become a serious threat in Scandinavia and Switzerland[16] and the first outbreak in Denmark was reported.[17] In autumn 2008, several cases were reported in the southern Swedish provinces of Småland, Halland, and Skåne,[18] as well as in areas of the Netherlands bordering Germany, prompting veterinary authorities in Germany to intensify controls.[19] Norway saw its first finding in February 2009, when cows at two farms in Vest-Agder in the south of Norway showed an immune response to bluetongue.[20] Norway have since been declared free of the disease in 2011.

Although the disease is not a threat to humans the most vulnerable common domestic ruminants in the UK are cattle, goats and, especially, sheep.

A puzzling aspect of BTV is its survival between midge seasons in temperate regions. Adult Culicoides are killed by cold winter temperatures, and BTV infections typically do not last for more than 60 days, which is not long enough for BTV to last until the next spring. It is believed that the virus somehow survives in overwintering midges or animals. Multiple mechanisms have been proposed. A few adult Culicoides midges infected with BTV may survive the mild winters of the temperate zone. Some midges may even move indoors to avoid the cold temperature of the winter. Additionally, BTV could cause a chronic or latent infection in some animals, providing another means for BTV to survive the winter. BTV can also be transmitted from mother to fetus. The outcome is abortion or stillbirth if fetal infection occurs early in gestation and survival if infection occurs late. However infection at an intermediate stage, before the fetal immune system is fully developed, may result in a chronic infection that lingers until the first months after birth of the lamb. Midges will then spread the disease from the calves to other animals, starting a new season of infection.[21]

However, simple husbandry changes and practical midge control measures may help break the livestock infection cycle. Housing livestock during times of maximum midge activity (from dusk to dawn) may lead to significantly reduced biting rates. Similarly, protecting livestock shelters with fine mesh netting or coarser material impregnated with insecticide will reduce contact with the midges. The Culicoides midges that carry the virus usually breed on animal dung and moist soils, either bare or covered in short grass. Identifying breeding grounds and breaking the breeding cycle will significantly reduce the local midge population. Turning off taps, mending leaks and filling in or draining damp areas will also help dry up breeding sites.[22] Control by trapping midges and removing their breeding grounds may reduce vector numbers. Dung heaps or slurry pits should be covered or removed, and their perimeters (where most larvae are found) regularly scraped.[23]

Outbreaks in southern Europe have been caused by serotypes 2 and 4, and vaccines are available against these serotypes (ATCvet codes: QI04AA02 for sheep, QI02AA08 for cattle). However, the disease found in northern Europe (including the UK) in 2006 and 2007 has been caused by serotype 8. Vaccine companies Fort Dodge Animal Health (Wyeth), Merial and Intervet were developing vaccines against serotype 8 (Fort Dodge Animal Health has serotype 4 for sheep, serotype 1 for sheep and cattle and serotype 8 for sheep and cattle) and the associated production facilities. A vaccine for this is now available in the UK, produced by Intervet. Fort Dodge Animal Health has their vaccines available for multiple European Countries (vaccination will start in 2008 in Germany, Belgium, Switzerland, Spain and Italy). However, immunization with any of the available vaccines preclude later serological monitoring of affected cattle populations, a problem which could be resolved using next-generation subunit vaccines currently in development. [24]

In January 2015, Indian researchers launched its vaccine. Named 'Raksha Blu', it will protect the animals against five strains of the ‘bluetongue’ virus prevalent in the country.[25]

Although bluetongue disease was already recognized in South Africa in the early 19th century, a comprehensive description of the disease was not published until the first decade of the 20th century.[26] In 1906 Arnold Theiler showed that bluetongue was caused by a filterable agent. He also created the first bluetongue vaccine, which was developed from an attenuated BTV strain.[27] For many decades bluetongue was thought to be confined to Africa. The first confirmed outbreak outside of Africa occurred in Cyprus in 1943.[26]

African horse sickness is related to Bluetongue and is spread by the same midges (Culicoides species). It can kill the horses it infects and mortality may go as high as 90% of the infected horses during an epidemic.[28]